The present disclosure relates to a biosensor system for measuring an analyte in a bodily fluid, such as blood, wherein the system comprises processes and systems for the deposition of reagents into multiple well biosensors. For example, the present disclosure provides methods of applying a chemistry solution within a well (e.g. sample cavity, sample chamber, or capillary), of a biosensor to enable the measurement of a specific analyte of a blood sample such as, for example, blood ketones, hemoglobin A1c, cholesterol, hematocrit, and triglicerides. Further, the biosensor system may contain additional wells to enable the measurement of additional blood analytes. While described herein in relation to blood constituent testing, the invention can be used to measure analytes in other fluid samples as well.
Electrochemical sensors have long been used to detect and/or measure the presence of substances in a fluid sample. In the most basic sense, electrochemical sensors comprise a reagent mixture containing at least an electron transfer agent (also referred to as an “electron mediator”) and an analyte specific bio-catalytic protein (e.g. a particular enzyme), and one or more electrodes. Such sensors rely on electron transfer between the electron mediator and the electrode surfaces and function by measuring electrochemical redox reactions. When used in an electrochemical biosensor system or device, the electron transfer reactions are transformed into an electrical signal that correlates to the concentration of the analyte being measured in the fluid sample.
The use of such electrochemical sensors to detect analytes in bodily fluids, such as blood or blood-derived products, tears, urine, or saliva, has become important, and in some cases, vital, to maintain the health of certain individuals. In the health care field, people such as diabetics, for example, have a need to monitor a particular constituent within their bodily fluids. A number of systems are available that allow people to test a body fluid, such as, blood, urine, or saliva, to conveniently monitor the level of a particular fluid constituent, such as, for example, cholesterol, proteins, and glucose. Patients suffering from diabetes, a disorder of the pancreas where insufficient insulin production prevents the proper digestion of sugar, have a need to carefully monitor their blood glucose levels on a daily basis. Routine testing and controlling blood glucose for people with diabetes can reduce their risk of serious damage to the eyes, nerves, and kidneys.
A number of systems permit people to conveniently monitor their blood glucose levels, and such systems typically include a test strip where the user applies a blood sample and a meter that “reads” the test strip to determine the glucose level in the blood sample. An exemplary electrochemical biosensor is described in U.S. Pat. No. 6,743,635 ('635 patent) which is incorporated by reference herein in its entirety. The '635 patent describes an electrochemical biosensor used to measure glucose level in a blood sample. The electrochemical biosensor system is comprised of a test strip and a meter. The test strip includes a sample chamber, a working electrode, a counter electrode, and fill-detect electrodes. A reagent layer is disposed in the sample chamber. The reagent layer contains an enzyme specific for glucose, such as, glucose oxidase, and a mediator, such as, potassium ferricyanide or ruthenium hexaamine. When a user applies a blood sample to the sample chamber on the test strip, the reagents react with the glucose in the blood sample and the meter applies a voltage to the electrodes to cause redox reactions. The meter measures the resulting current that flows between the working and counter electrodes and calculates the glucose level based on the current measurements.
Biosensors configured to measure a blood constituent may be affected by the presence of certain blood components that may undesirably affect the measurement and lead to inaccuracies in the detected signal. This inaccuracy may result in an inaccurate glucose reading, leaving the patient unaware of a potentially dangerous blood sugar level, for example. As one example, the particular blood hematocrit level (i.e. the percentage of the amount of blood that is occupied by red blood cells) can erroneously affect a resulting analyte concentration measurement.
Variations in a volume of red blood cells within blood can cause variations in glucose readings measured with disposable electrochemical test strips. Typically, a negative bias (i.e., lower calculated analyte concentration) is observed at high hematocrits, while a positive bias (i.e., higher calculated analyte concentration) is observed at low hematocrits. At high hematocrits, for example, the red blood cells may impede the reaction of enzymes and electrochemical mediators, reduce the rate of chemistry dissolution since there less plasma volume to solvate the chemical reactants, and slow diffusion of the mediator. These factors can result in a lower than expected glucose reading as less current is produced during the electrochemical process. Conversely, at low hematocrits, less red blood cells may affect the electrochemical reaction than expected, and a higher measured current can result. In addition, the blood sample resistance is also hematocrit dependent, which can affect voltage and/or current measurements.
Several strategies have been used to reduce or avoid hematocrit based variations on blood glucose readings as described in U.S. patent application Ser. No. 11/401,458 which is incorporated by reference herein in its entirety. For example, test strips have been designed to incorporate meshes to remove red blood cells from the samples, or have included various compounds or formulations designed to increase the viscosity of red blood cell and attenuate the effect of low hematocrit on concentration determinations. Further, biosensors have been configured to measure hematocrit by measuring optical variations after irradiating the blood sample with light, or measuring hematocrit based on a function of sample chamber fill time. These methods have the disadvantages of increasing the cost and complexity of test strips and may undesirably increase the time required to determine an accurate glucose measurement.
In addition, alternating current (AC) impedance methods have also been developed to measure electrochemical signals at frequencies independent of a hematocrit effect. Such methods suffer from the increased cost and complexity of advanced meters required for signal filtering and analysis.
An additional prior hematocrit correction scheme is described in U.S. Pat. No. 6,475,372. In that method, a two potential pulse sequence is employed to estimate an initial glucose concentration and determine a multiplicative hematocrit correction factor. A hematocrit correction factor is a particular numerical value or equation that is used to correct an initial concentration measurement, and may include determining the product of the initial measurement and the determined hematocrit correction factor. Data processing using this technique, however, is complicated because both a hematocrit correction factor and an estimated glucose concentration must be determined to establish the corrected glucose value. In addition, the time duration of the first step greatly increases the overall test time of the biosensor, which is undesirable from the user's perspective.
A further hematocrit correction method is described in U.S. Patent Application No. 60/842,032 filed Sep. 5, 2006, which is incorporated by reference herein in its entirety. In particular, the concept of a low blood volume sensor with multiple sample cavities filled with a single blood drop is disclosed. This arrangement allows for the measurement of multiple analytes within a blood sample, such as hematocrit, in addition to measuring the glucose level. Thus, a corrected glucose level can be determined by taking into account the levels of the other analytes measured in the blood sample.
The measurement of multiple analytes, however, may require the application of different chemistry solutions in various sample cavities. For example, when one of the sample cavities is used for hematocrit measurement by measuring the resistance of the blood sample, it is desirable to measure this resistance in whole blood without any chemical additives in it in order to minimize the background effect from the electrolytes from the reagent chemistry. In contrast, other sample cavities may contain biosensor chemistry solutions containing enzymes specific for the analyte of interest, such as mediators, binders, stabilizers and surfactants. While the actual placement of biosensor chemistry solutions, or biosensor reagents, can be accomplished by means of precision dispensing and machine vision, drop spreading after dispensing is controlled largely by the dimensions and surface properties of the sensor itself and the properties of the biosensor reagent solution.
Typically, a biosensor reagent is formulated with a large amount of the surfactant, to ensure uniform spreading and fast dissolution of the dried chemistry layer upon contact with the blood sample. The presence of surfactants in the biosensor reagent, however, makes it difficult to reproducibly dispense the reagent within a certain area, since the surfactant promotes spreading of the biosensor reagent into an adjacent sample cavity. On the other hand, reducing the amount of surfactant below the optimal concentration results in a non-uniform coating and a negative impact on measurement precision. Further, traditional methods for biosensor reagent application, such as screen printing, rotogravure or flexo printing are not easily applied to the multi-well sensor because of registration issues associated with applying two or more chemistry patterns within a very small area of the sensor.
Accordingly, novel systems and methods for reproducibly and selectively applying a biosensor reagent to a single sample cavity, within a multiple sample cavity biosensor, are needed.